I knew about the split image focusing system, but I didn't know about the insensitivity to the aperture control (despite it might sound obvious, since you don't see through the lens... but since you can see how you're focusing...).

So, I guess it takes very high skills to shoot with a rangefinder!

The focusing is done through a coupling cam driven by the lens mount. The rangefinder mechanism is calibrated to the focal length of the lens. When you mount the lens it tells the mechanism which one is on.